The invention relates to lamps and, in particular, to LED tube lamps which have one or more LEDs as light sources and which can replace a fluorescent tube.
Fluorescent lamps are widely used in different environments, such as in homes, offices and industry. Fluorescent lamps are more durable, economical and efficient than incandescent lamps, in which most of the electric power generates heat rather than light. In a conventional fluorescent lamp, the body is a straight tube with a length of 15 to 60 times the diameter of the tube. The tube may also be bent, in which case it may be of almost any shape. Fluorescent tubes are low-pressure mercury discharge lamps in which the inner surface of the tube is coated with a fluorescent material. The structure of a fluorescent tube is very simple and is illustrated in FIG. 1A. The lamp consists of an airtight glass tube 4 containing a small amount of mercury, an inert gas, a fluorescent coating (luminophor), such as phosphor, as well as electrodes 2 and 3 (filaments). At each end of the fluorescent tube, there is a cap 5 or 6 with two symmetrically positioned contact pins 7 and 8 or 9 and 10, to which the electrode 2 or 3 is connected. The power supply to the fluorescent tube is provided via these contact pins 7 and 8; 9 and 10. When the lamp is in operation, the temperature of the electrodes 2 and 3 must be sufficiently high in order to enable electrons to be released from them. A fluorescent lamp does not go on at a normal operating voltage without preheating. It is typical of fluorescent tubes (EN 60081) that their cathodes are heated with separate preheat circuits or arrangements. On the other hand, after the lamp has gone on, the discharging current through the tube must be restricted, so that the tube will not be damaged. Therefore, all fluorescent tubes require a ballast. Conventionally, the ballast has been a ballast-starter combination, which is illustrated in FIG. 1B. When a mains voltage (e.g. 230 VAC) is connected to the lighting fixture, the resistance through the tube is very high, and the electric current passes through a ballast L, the electrode 3, a closed starter 11 and the electrode 2. When passing through the electrodes 2 and 3, the electric current heats the electrodes, causing them to emit electrons which ionize the gas inside the tube. The ionized gas forms a current path through the tube. The current passing through the ballast L generates a magnetic field in the ballast. When, after a moment, the starter 11 opens, the magnetic field of the ballast L generates a high voltage between the electrodes 2 and 3, which switches the lamp on.
Nowadays, electronic ballasts are also used. The electronic ballast also attends to switching the lamp on, so there is no need for a separate starter. A preheating arrangement is implemented by either separate pre-heating windings or a starter capacitor. This is illustrated in FIG. 1C. An electronic ballast 12 connected to the mains voltage (e.g. 230 VAC) provides continuous electric current through each of the electrodes 2 and 3. These electric currents are configured in such a way that a voltage difference is generated between the electrodes 2 and 3. When the mains voltage is connected to the ballast 12, the electric current passing through the electrodes heats them quickly, and the emitted electrons ionize the gas in the tube. The gas having ionized, the voltage difference between the electrodes starts a gas discharge.
A common aim is to replace fluorescent tubes with LED tubes having the same length and values. Their physical dimensions are the same as those of straight fluorescent tubes (e.g. T8 with a diameter of 26 mm and a length of 60 or 120 cm), whereby the fluorescent tube could be directly replaced with a LED tube in an existing fluorescent tube lighting fixture. Examples of LED tube lamps directly connected to the mains voltage with a ballast are disclosed in publications EP1852648 and U.S. Pat. No. 7,441,922. Examples of LED tube lamps provided with an electronic ballast are disclosed in publications FI64487 and US2007/0183156. Here, the electronic ballast usually supplies a high-frequency (20 kHz . . . 100 kHz) voltage to the fluorescent tube pins, and the control electronics of the LEDs carry out rectification and current limiting for the LEDs to the appropriate extent. Other examples of LED tube lighting fixtures are disclosed in publications US2010/0002439 and WO2009/131340. The aim is to achieve a long lifetime for the light source as well improved luminous efficiency (quantity of light/electric power).
In practice, the intention is to replace a fluorescent tube with a LED tube without changing structures of the lighting fixture. Some of the LED tubes work directly with a fluorescent tube ballast, in which case only the starter is to be removed from service. Then, the LED tube can be replaced easily and without assistance from an expert.
This causes a few problems, the most significant of which is a risk of an electric shock when the LED tube is being installed. FIG. 2 shows a simplified conceptual drawing of a fluorescent tube lighting fixture 20 comprising a body 24 containing the required electric structures, such as the ballast 12 and the starter 11, which is usually required only in connection with a ballast. At the ends of the lighting fixture, there are tube holders 21 and 22 with contact sockets 23 into which contact pins of ends 26 and 27 of a tube 25 are inserted to achieve mechanical and electric connection. According to the safety regulations in the field of electricity, lighting fixtures are to be constructed in such a way that when a fluorescent tube is replaced, it is impossible for a user to come into contact with any parts at the mains voltage even if the lighting fixture were connected to the mains voltage. This requirement is met even if the fluorescent tube were replaced in such a way that only one end 27 of the tube 25 is in contact with the contacts 23 of the tube holder 22 while the other end 26 of the tube may come into contact with the person replacing the tube. The requirement is met because no current is passing through the gas-filled fluorescent tube before the gas in the tube is ionized with a starting pulse. In other words, the gas in the fluorescent tube serves as an insulator in itself. The electric structure of the lighting fixture, in turn, is such that generation of a starting pulse requires that both ends of the tube be connected to the contacts of the tube holder. Thus, it has become possible to avoid a risk of an electric shock in connection with a fluorescent tube when replacing the tube.
In connection with LED tubes, this electric safety requirement is not met. Inside LED tubes, there is a printed board or the like structure on which the LEDs and the required electronic current supply components are installed. The purpose of these components is to convert the alternating voltage of the mains into direct voltage and to regulate the direct current required by the LEDs. In practice, current starts flowing through these components once voltage is applied to them, in other words the LED tube is in a conductive state without any separate starting pulse. Therefore, in practice, when the LED tube 25 is being installed in a fluorescent tube lighting fixture 20, the contact pins 27 at one end of the LED tube 25 may hit the contacts 23 of the tube holder 22, and the other end 26 of the tube may remain outside the lighting fixture so that the person installing or replacing the tube may come into contact with it, whereby he/she is subjected to the risk of an electric shock.
Another factor deteriorating electric safety is the cooling of a LED tube. Since the service life of LEDs is highly dependent on their operating temperature, various solutions have been sought for cooling the LED tube. Some solutions suggest perforation of the LED tube (e.g. U.S. Pat. No. 7,611,260), in which case air flows through the holes, transferring heat out of the LEDs to the outside of the tube. In such a solution, owing to the plastic body of the LED tube, high insulation level of the energized parts can still be maintained.
Another cooling solution is disclosed in publications EP2151620 and US 2007/0183156, where part of the LED tube is of metal which serves as a good heat conductor, transferring heat out of the LEDs. A problem with these cooling solutions using metal is that the metal cooling part must be insulated sufficiently reliably from the LED circuits. Thus, sufficient insulation distances are required. If such LED tubes with metal cooling parts are used in such a way that they are fed by an electronic ballast, a further problem is caused by a high frequency, in particular. That is to say, the conductor circuits of the LEDs generate distributed capacitance in said metal structure, which generates a capacitive leakage current. This leakage may cause a risk of an electric shock which may even be life-threatening.